Assays using label independent detection (LID) platforms (e.g. surface plasmon resonance (SPR) or resonant waveguide grating sensors) are typically performed using a two step procedure: (i) immobilization of one of the binding partners (e.g., a protein) on the surface of the sensor; and (ii) binding of a ligand (e.g., drug, protein, oligonucleotide, etc) to the immobilized protein. Traditionally, the coupling of biomolecules to surfaces involves the activation of carboxylic acid groups on the surface to reactive N-hydroxysuccinimide (NHS) esters, which are then coupled to amino groups on the protein of interest. This method has been successfully used and commercialized by Biacore, Affinity Biosensors, and Artificial Sensing Instruments for their respective LID platforms. While effective, the activation step is time consuming and involves the handling and use of somewhat toxic chemicals.
An alternative to this approach involves the use of “preactivated” chemistries. For example, surfaces presenting aldehyde groups have been used to bind biomolecules. However, a reduction step is required after coupling to stabilize the resulting Schiff base. Surfaces with epoxide and isocyanate functionalities have also been used; however, the epoxide group is relatively slow to react and, therefore, requires long incubation times under very basic conditions, while the isocyanate group is extremely reactive and presents storage stability issues. Because of these issues, there are few reports of the use of preactivated chemistries for LID platforms. In fact, neither Biacore, Affinity Biosensors, nor ASI—the three companies offering the most popular LID platforms—offer sensors with a preactivated chemistry.
Maleic anhydride reacts readily with nucleophiles such as amino groups. Although the modification of surfaces with maleic anhydride copolymer layers for the immobilization of small molecules, DNA, sugars, and peptides has been described, the hydrolytic stability of maleic anhydrides is rather poor, and for this reason they have not been widely used. The hydrolytic stability of maleic anhydride can be increased when copolymerized with hydrophobic side chains (e.g. styrene); however, this leads to problems with nonspecific binding of biomolecules to the surface. While this may be an advantage for some applications such as mass spectrometry, it is problematic for LID.
There is a unique issue with LID detection in general that necessitates a stringent requirement for biospecificity. The incorporation of “blocking agents” (e.g. bovine serum albumin, BSA) in the analyte solution is undesirable because both specific (due to the analyte) and non-specific (due to the blocking agent) binding would contribute to changes in interfacial refractive index and would hence be indistinguishable. This problem is only exacerbated when complex samples are used or when the analyte is impure. The concern with anhydrides for immobilization of biomolecules such as, for example, proteins is non-specific binding due to the formation of residual negative charge and the influence of other groups (e.g. styrene, ethylene, methyl vinyl ether, etc) in the polymer. Because of these reasons, the feasibility of using anhydride polymers for LID is a potential concern.
The supports and methods described herein provide numerous advantages. For example, the support does not need to be activated, which saves the user time, cost, and complexity. The supports and methods described herein permit the loading of high amounts of biomolecules, which leads to better sensitivity with respect to detecting an analyte. Additionally, the methods for producing the supports permit high-volume manufacturing of the supports. In general, the supports are stable and can be stored for extended (˜6 months) periods of time with little or no loss in binding capacity. Moreover the coated substrates are slow to hydrolyze under acidic conditions, which permits the binding of various biomolecules under conditions that have not been described using prior art techniques for polymers such as, for example anhydride polymers. Finally, the supports and methods also increase array signal intensity, sensitivity and assay quality in a timely and economical manner and further improve the assay specificity.